Automatically actuated clutches of this type may be used on vehicles equipped with various transmissions. The differentiation of the transmission in this context involves not just the degree of automation, but also the type of design, so that the transmission may be, for example, an automated shift transmission, an uninterrupted shift transmission, a parallel shift transmission or a transmission having continuously variable change of the gear ratio. In such transmissions the clutch, in the form of a friction clutch, generally performs two functions, namely, the function of acting as a starting element and also the function of acting as a shift element.
Both when starting and when reestablishing the power transmission connection between the engine and the drive train after a gear change operation, a clutch torque is transmitted via the clutch when there is a speed differential between the input and output sides of the friction clutch. In this context, friction loss is inevitably brought into the clutch frictional surfaces as a product of clutch torque and differential angular velocity, which causes a temperature increase in the frictional surfaces and thus the clutch.
The wear of the clutch surfaces is a function of the amount of input energy and the temperature. In addition to a proper wear of the friction surfaces, there may also be secondary effects involved in the form of mechanical deformations of the friction surfaces and changes of the coefficient of friction of the friction surfaces, so that, for example, a drop in the coefficient of friction of the friction counterparts may result in an extension of the slip phase and, thus, in turn, an increase of the input energy.
It is therefore of fundamental significance, especially with dry clutches, not to let the input of energy in the clutch become too high.
In vehicles with automatically actuated clutches, the direct actuation of the clutch is taken away from the influence of the driver, but the driver through his driving style may have an influence on the wear behavior of the clutch.
Therefore, it is possible, for example, that a long-lasting creeping of the vehicle on an incline that is noticed by the driver, or even unnoticed, or a delayed starting behavior on an incline leads to an increased energy input in the clutch. Also, stopping the vehicle on an incline by using the accelerator pedal leads to an increased energy input into the clutch, this also being applicable, for example, for the case that the driver forgets that his parking brake is engaged when starting to drive.
In addition to these causes of increased energy input in the clutch that are brought about by the driver of the vehicle, errors in the system for actuating the automatic friction clutch may result in increased energy input, for example, because of a hydraulic line for clutch actuation being out of tune, due, for example, to a leak. These are all just examples of causes of an increased load in the form of an increased energy input in the friction clutch.